It is now common practice to provide agricultural drills with tramlining devices that operate to leave selected rows unsown in selected bouts so as to produce tracks or tramlines in the subsequent crop for the passage of a vehicle when treating the crop such as by spreading or spraying fertilizer or insecticide.
Generally, the subsequent crop treatment covers two or more drill bouts in one pass of the treatment vehicle, and the treated width conveniently coincides with the edges of the drill bouts. However, a situation can arise in which the treated width does not coincide with the edges of the drill bouts and half of the first drill bout will not be treated by the first pass of the treatment vehicle. An agricultural drill has therefore been proposed in West German Patent Specification No. DE-2924625 to allow half a bout width to be sown. Said drill has a plurality of seed dispensing units that are spaced across the width of the drill and are driven by a drive mechanism comprising two coaxial drive shafts each extending over one half the width of the drill and each being selectively driveable so as to sow half a width either side of the centre line of a drill bout or to sow a full width bout. Each drive shaft is driven through a corresponding clutch mechanism at the centre of the drill comprising a first gearwheel on the shaft, a second gearwheel on a pivoted arm that is operated to engage or disengage said first and second gearwheels, and a third gearwheel in constant mesh with the second gearwheel and which is carried on an input drive shaft coaxial with the pivoted arms. Thus, two clutch mechanisms are provided to control drive to the adjacent inner ends of both seed unit drive shafts, and an input drive shaft transfers drive from the side of the drill to these clutches.
Agricultural drills are also known, for example as shown in U.S. Pat. No. 3,620,419, in which a plurality of seed dispensing units are driven by a common drive shaft that is axially adjustable to vary the feed rate of the dispensing units.
An object of the present invention is to provide an agricultural drill having a drive mechanism for the seed dispensing units which is adapted to allow either a full or half a bout width to be sown but which is simpler than the known drive mechanism of this type employing two coaxial drive shafts each associated with a respective half of the drill width. A further object is to provide that said drive mechanism is adapted so that it allows adjustment of the feed rate of the seed dispensing units through axial adjustment of said drive shafts driving said units.